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promises a promises a an open standard for sound interoperable javascript promises by implementers for implementers a promise represents the eventual result of an asynchronous operation the primary way of interacting with a promise is through its then method which registers callbacks to receive either a promise s eventual value or the reason why the promise cannot be fulfilled this specification details the behavior of the then method providing an interoperable base which all promises a conformant promise implementations can be depended on to provide as such the specification should be considered very stable although the promises a organization may occasionally revise this specification with minor backward compatible changes to address newly discovered corner cases we will integrate large or backward incompatible changes only after careful consideration discussion and testing historically promises a clarifies the behavioral clauses of the earlier promises a proposal extending it to cover de facto behaviors and omitting parts that are underspecified or problematic finally the core promises a specification does not deal with how to create fulfill or reject promises choosing instead to focus on providing an interoperable then method future work in companion specifications may touch on these subjects terminology promise is an object or function with a then method whose behavior conforms to this specification thenable is an object or function that defines a then method value is any legal javascript value including undefined a thenable or a promise exception is a value that is thrown using the throw statement reason is a value that indicates why a promise was rejected requirements promise states a promise must be in one of three states pending fulfilled or rejected when pending a promise may transition to either the fulfilled or rejected state when fulfilled a promise must not transition to any other state must have a value which must not change when rejected a promise must not transition to any other state must have a reason which must not change here must not change means immutable identity i e but does not imply deep immutability the then method a promise must provide a then method to access its current or eventual value or reason a promise s then method accepts two arguments promise then onfulfilled onrejected both onfulfilled and onrejected are optional arguments if onfulfilled is not a function it must be ignored if onrejected is not a function it must be ignored if onfulfilled is a function it must be called after promise is fulfilled with promise s value as its first argument it must not be called before promise is fulfilled it must not be called more than once if onrejected is a function it must be called after promise is rejected with promise s reason as its first argument it must not be called before promise is rejected it must not be called more than once onfulfilled or onrejected must not be called until the execution context stack contains only platform code 3 1 onfulfilled and onrejected must be called as functions i e with no this value 3 2 then may be called multiple times on the same promise if when promise is fulfilled all respective onfulfilled callbacks must execute in the order of their originating calls to then if when promise is rejected all respective onrejected callbacks must execute in the order of their originating calls to then then must return a promise 3 3 promise2 promise1 then onfulfilled onrejected if either onfulfilled or onrejected returns a value x run the promise resolution procedure resolve promise2 x if either onfulfilled or onrejected throws an exception e promise2 must be rejected with e as the reason if onfulfilled is not a function and promise1 is fulfilled promise2 must be fulfilled with the same value as promise1 if onrejected is not a function and promise1 is rejected promise2 must be rejected with the same reason as promise1 the promise resolution procedure the promise resolution procedure is an abstract operation taking as input a promise and a value which we denote as resolve promise x if x is a thenable it attempts to make promise adopt the state of x under the assumption that x behaves at least somewhat like a promise otherwise it fulfills promise with the value x this treatment of thenables allows promise implementations to interoperate as long as they expose a promises a compliant then method it also allows promises a implementations to assimilate nonconformant implementations with reasonable then methods to run resolve promise x perform the following steps if promise and x refer to the same object reject promise with a typeerror as the reason if x is a promise adopt its state 3 4 if x is pending promise must remain pending until x is fulfilled or rejected if when x is fulfilled fulfill promise with the same value if when x is rejected reject promise with the same reason otherwise if x is an object or function let then be x then 3 5 if retrieving the property x then results in a thrown exception e reject promise with e as the reason if then is a function call it with x as this first argument resolvepromise and second argument rejectpromise where if when resolvepromise is called with a value y run resolve promise y if when rejectpromise is called with a reason r reject promise with r if both resolvepromise and rejectpromise are called or multiple calls to the same argument are made the first call takes precedence and any further calls are ignored if calling then throws an exception e if resolvepromise or rejectpromise have been called ignore it otherwise reject promise with e as the reason if then is not a function fulfill promise with x if x is not an object or function fulfill promise with x if a promise is resolved with a thenable that participates in a circular thenable chain such that the recursive nature of resolve promise thenable eventually causes resolve promise thenable to be called again following the above algorithm will lead to infinite recursion implementations are encouraged but not required to detect such recursion and reject promise with an informative typeerror as the reason 3 6 notes here platform code means engine environment and promise implementation code in practice this requirement ensures that onfulfilled and onrejected execute asynchronously after the event loop turn in which then is called and with a fresh stack this can be implemented with either a macro task mechanism such as settimeout or setimmediate or with a micro task mechanism such as mutationobserver or process nexttick since the promise implementation is considered platform code it may itself contain a task scheduling queue or trampoline in which the handlers are called that is in strict mode this will be undefined inside of them in sloppy mode it will be the global object implementations may allow promise2 promise1 provided the implementation meets all requirements each implementation should document whether it can produce promise2 promise1 and under what conditions generally it will only be known that x is a true promise if it comes from the current implementation this clause allows the use of implementation specific means to adopt the state of known conformant promises this procedure of first storing a reference to x then then testing that reference and then calling that reference avoids multiple accesses to the x then property such precautions are important for ensuring consistency in the face of an accessor property whose value could change between retrievals implementations should not set arbitrary limits on the depth of thenable chains and assume that beyond that arbitrary limit the recursion will be infinite only true cycles should lead to a typeerror if an infinite chain of distinct thenables is encountered recursing forever is the correct behavior to the extent possible under law the promises a organization has waived all copyright and related or neighboring rights to promises a promise specification this work is published from united states specification changelog credits differences from promises a implementations compliance tests github organization